Human natural killer (NK) cells and K cells mediating antibody-dependent cellular cytotoxicity have been shown to be large granular lymphocytes (LGL). The majority of LGL form lytic conjugates with a large panel of NK-susceptible target cells. Interferon causes augmentation of NK and K cell activities of LGL and only LGL demonstrated either spontaneous or interferon activated NK activity. The mechanism of augmentation appears to be multiple: 1) increase in lytic efficiency, 2) increase in lytic binders, 3) increase in number of binders, and 4) increased recycling of effector cells. Analysis of the phenotype of LGL with monoclonal antibodies demonstrated sharing of a variety of antigens with both T cells and monocyte cells. Cultures of highly purified LGL in the presence of T cell growth factor demonstrated typical morphology and cytotoxic patterns of fresh NK cells.